Conventionally, a strain sensor that detects a strain of a strain body has been used to detect a treading load of vehicular pedals.
FIG. 9 is a cross-sectional view illustrating a structure of a conventional strain sensor.
As illustrated in FIG. 9, conventional strain sensor 1 includes strain body 2, fixing member (first member) 3, and displacement member (second member) 4, which are arranged concentrically to one another. First strain resistance element (strain detection element) 5 is disposed on an outer surface at a lower part of strain body 2. One end of first strain resistance element 5 is electrically connected to a power supply electrode (not illustrated) through a circuit pattern (not illustrated). The other end of first strain resistance element 5 is electrically connected to a second output electrode (not illustrated). Second strain resistance element (strain detection element) 6 is disposed substantially in parallel with first strain resistance element 5, on an outer surface at the lower part of strain body 2. One end of second strain resistance element 6 is electrically connected to the power supply electrode through a circuit pattern (not illustrated). The other end of second strain resistance element 6 is electrically connected to a first output electrode (not illustrated).
Third strain resistance element (strain detection element) 7 is disposed on an outer surface at an upper part of strain body 2. One end of third strain resistance element 7 is electrically connected to first strain resistance element 5 and the second output electrode through a circuit pattern (not illustrated). The other end of third strain resistance element 7 is electrically connected to a GND electrode (not illustrated).
Further, fourth strain resistance element (strain detection element) 8 is disposed substantially in parallel with third strain resistance element 7 on the outer surface at the upper part of strain body 2. One end of fourth strain resistance element 8 is electrically connected to second strain resistance element 6 and the first output electrode through a circuit pattern. The other end of fourth strain resistance element 8 is electrically connected to the GND electrode. A full bridge circuit is configured as above.
Ferritic stainless steel fixing member (first member) 3 includes disk-shaped attachment part 9 and shaft part 10 integrally including attachment part 9 at an intermediate part in a longitudinal direction. An outer circumferential part of attachment part 9 is welded to strain body 2 while being engaged with a side edge of strain body 2, in a state in which attachment part 9 blocks one end opening of strain body 2. One end part of shaft part 10 of fixing member 3 penetrates through an inner side of strain body 2. Displacement member (second member) 4 made of metal (for example, ferritic stainless steel) is configured with annular washer 11 and cylindrical attachment member 12 for functioning as a case that is fixed to one end of washer 11. At an inner side of attachment member 12, an outer circumferential part of washer 11 is fixed to an opening edge of the other end part of strain body 2 by welding, while being engaged with strain body 2. Cylindrical attachment member 12 for functioning as the case accommodates attachment part 9, strain body 2, and washer 11.
In conventional strain sensor 1 illustrated in FIG. 9, a load is applied to displacement member 4 in a direction perpendicular to shaft center A in strain body 2, and therefore shearing force is applied to strain body 2 (PTL 1).
Subsequently, another strain sensor will be described with reference to FIG. 10.
FIG. 10 is a top view of the other conventional strain sensor.
As illustrated in FIG. 10, strain sensor 21 is configured with insulation substrate 22 and a bridge circuit. Further, the bridge circuit is configured such that power supply electrode 23, a pair of output electrodes 24, and GND electrode 25, which are made of silver and are provided on an upper surface of insulation substrate 22, and four strain resistance elements 26 are electrically connected to one another through circuit patterns 27.
At least a pair of temperature characteristic adjusting resistors 28 is disposed on the upper surface of insulation substrate 22. One ends of temperature characteristic adjusting resistors 28 each are electrically connected to power supply electrode 23, and the other ends of temperature characteristic adjusting resistors 28 each are electrically connected to strain resistance elements 26 through a pair of resistance value measuring electrodes 29. In addition, frame GM) electrode 30 is disposed on the upper surface of insulation substrate 22. Furthermore, capacitor 31 and electrostatic discharge resistor 32 are electrically connected in parallel with each other between frame GM) electrode 30 and GM) electrode 25 through circuit patterns 27. Slit part 33 is disposed in circuit patterns 27 of insulation substrate 22 to disconnect a part of one of circuit patterns 27. Four strain resistance elements 26 disposed on the upper surface of insulation substrate 22 are divided into two pairs. Further a portion between two strain resistance elements 26 included in each of two divided pairs of strain resistance elements 26 in insulation substrate 22 is made thinner to configure thin width part 22a. 
When shearing load is applied to a center part of insulation substrate 22, a strain is generated on a surface of insulation substrate 22 due to the shearing load. Further, strains are also generated in four strain resistance elements 26 disposed on the upper surface of insulation substrate 22. The strains generated in strain resistance elements 26 each change resistance values of corresponding strain resistance elements 26. Then, the changes in the resistance values of strain resistance elements 26 are output from the pair of output electrodes 24 to an external computer (not illustrated), and thus a load applied to insulation substrate 22 can be measured (PTL 2).